The present invention relates to a method of charging a secondary battery by means of a constant voltage power source for supplying a limited current, and more particularly to prevent overcharge.
A nonaqueous electrolyte secondary battery of rechargeable secondary batteries provides a high battery voltage (e.g., 4.1 V), a high energy density, and a superior cycle characteristic. For these advantages, the nonaqueous electrolyte secondary battery is used as a memory backup or a power source for various electronic equipments. Such a nonaqueous electrolyte secondary battery may be constructed by employing a carbonaceous material as a negative electrode, a lithium compound as a positive electrode, and a nonaqueous electrolytic solution of an electrolyte in a nonaqueous solvent as a nonaqueous electrolyte.
As a method of charging a secondary battery such as the above-mentioned nonaqueous electrolyte secondary battery, there is known a charging method using a constant voltage power source for supplying a limited current (which will be hereinafter referred to as a current limiting constant voltage power source). According to the charging method using the current limiting constant voltage power source, a maximum current and a maximum voltage of the power source can be decided regardless of a discharging amount (remaining charge capacity) of the battery. Accordingly, a charging circuit in a battery charger can be simplified in construction.
FIG. 7 shows a charging pattern schematically representing a charging curve in case of charging a nonaqueous electrolyte secondary battery by means of the current limiting constant voltage power source as mentioned above.
In the case that a battery voltage of the nonaqueous electrolyte secondary battery is lower than a maximum voltage of the current limiting constant voltage power source, a charging current I is supplied from the power source to the battery with a maximum current I.sub.p limited at a constant value, thereby charging the battery. During the supply of the constant maximum current I.sub.p, a battery voltage V gradually rises.
When the battery voltage V reaches a maximum voltage V.sub.p of the power source, the maximum voltage V.sub.p is limited at a constant value to proceed with subsequent charging. At the same time, the charging current I starts lowering from the maximum current I.sub.p.
In such a charging method, overcharge of the battery may be avoided by ending the charging after a predetermined period T.sub.0 of time has elapsed from a charge start timing.
The maximum voltage of the current limiting constant voltage power source for charging the nonaqueous electrolyte secondary battery may be set to 4.1 V, for example, and the maximum current of the power source may be set to normally about 0.1-1.0 C (nominal capacity or rated capacity) amperes depending on a charging time (a period of time for fully charging the battery).
FIG. 8 shows a charging pattern schematically representing a charging curve in case of charging a nickel-cadmium secondary battery according to a conventional method.
In the case that the nickel-cadmium secondary battery is charged by a battery charger including a constant current power source, a constant charging current I flows in the battery, and a battery voltage V gradually rises to reach a peak voltage V.sub.p ', thereafter lowering. During the lowering of the battery voltage V, the battery charger detects the time when the battery voltage V becomes (V.sub.p '-.DELTA.V), and stops the charging. In this case, to compensate a self-discharge amount while the battery is in connection with the battery charger, a minute charging current as shown by a dashed line in FIG. 8 may be successively supplied.
In this way, overcharge of the nickel-cadmium secondary battery is prevented. Such measures for preventing the overcharge are effective particularly in case of quickly charging the nickel-cadmium secondary battery with a large current. That is, the nickel-cadmium secondary battery usually includes a mechanism for absorbing a gas to be generated by decomposition of an electrolytic solution upon overcharging. However, in case of charging the battery with a large current, there is a possibility that the gas cannot be sufficiently absorbed by the absorbing mechanism.
On the other hand, the nonaqueous electrolyte secondary battery does not include such an absorbing mechanism for absorbing a gas to be generated by decomposition of an electrolytic solution due to overcharge. Accordingly, if an overcharged condition of the battery continues, there is a possibility that a life of the battery is shortened.
As apparent from the charging pattern of the nonaqueous electrolyte secondary battery shown in FIG. 7, the charging current of the battery does not become zero, but continues to flow with a minute current even after the battery voltage becomes the maximum voltage of the power source. This is due to the fact that the decomposition of the nonaqueous electrolytic solution occurs minutely in the battery to consume a minute current. If this condition continues, the battery is overcharged.
To prevent such overcharge, as previously mentioned, there has been conventionally proposed a technique of stopping the charging after the predetermined period T.sub.0 has elapsed from the charge start timing. However, this technique has the following problem.
That is, the predetermined charging period T.sub.0 is decided on the basis that a charge capacity of the battery hardly remains in the battery. Therefore, if a battery with a charge capacity still remaining is charged by the aforementioned method, the battery is overcharged because it is always charged for the predetermined period T.sub.0.
In practice, charging of a battery with a remaining charge capacity often happens, and it is therefore important to prevent the aforementioned overcharge.